Interconnect rod for sucker rod string

Abstract

A device for coupling a driver system to a downhole string of sucker rods in a petroleum well employs a polish rod construction in which the polish rod is strengthened by an attached sleeve providing a transverse should in a predetermined plane. The sleeve is shrink fit in place and mechanically adhered to withstand the stresses and shocks encountered in long-term cyclic operation. The device provides specific benefits for an improved sucker rod connection which prestresses the sucker rod pin ends and the coupling in a fashion to resist the development of microcracks and fatigue failures.

Description

REFERENCE TO PRIOR APPLICATION[0001]This invention relies for priority on previously filed provisional application No. 60 / 837,563, filed Aug. 14, 2006 by Kenneth J. Carstensen and entitled “Interconnect Rod for Sucker Rod String”.BACKGROUND OF THE INVENTION[0002]A sucker rod string in a petroleum well extends from a power drive at the wellhead, down through the lengths of tubing and concentric casing used in a production system, and connects to the fluid pump at the lowest elevation in the system. The pump receives an accumulating petroleum inflow from the production zone somewhere above it and lifts a column of fluid up through the tubing. The power drive at the wellhead may be a reciprocating beam system, often called a horsehead, and the sucker rod string then reciprocates a downhole plunger pump which lifts fluid upward through the tubing. The power source may alternatively be a rotary drive coupled via the sucker rod string to a longitudinally extended progressing (or “progress...

AI Technical Summary

Benefits of technology

[0008]An improved polish rod system in accordance with the invention is configured for precise dimensional positioning and structural interaction with a sucker rod system. It provides the structural properties needed and an extremely durable construction for load cycling operation. The improved polish rod integrates a shoulder surface at a position that absorbs stresses both static and dynamic, transmitted along the sucker rod string and in the rod couplings. This improvement enables the connection to take advantage of prestressing, with or without an intermediate torque button.

[0009]A polish rod in accordance with the invention is externally configured to have the needed principal length and body diameter for the stress on the sucker rod string to which it is to be attached. The polish rod body is machined, ground and polished and spray coated with hardened material in accordance with prior practice, but includes a small diametral reduction for a predetermined length adjacent its lower end that has precisely defined end transitions. A shoulder fitting or sleeve of slightly smaller internal dimension than the reduced rod dimension but of the predetermined length is attached on the polish rod body, after being heated sufficiently to shrunk into place with secure engagement. A corner radius of curvature on the upper end of the shoulder sleeve engages a matched curvature at the corresponding end of the predetermined length of the polish rod, to aid in seating and retention. The sleeve includes a tapered section that diverges from its upper end to a mid-region transition and thereafter is of uniform cross section to its lower end. This end defines a transverse end shoulder in a transverse plane that is precisely aligned longitudinally with the end of a pin neck section in the polish rod. The pin neck provides stress relief and adjoins the terminating straight thread section which is compatible with the female thread in a sucker rod coupling. When the pin end is fully in threaded position in the coupling, the coupling end engages the transverse shoulder surface of the sleeve.

Problems solved by technology

It became evident that the polish rod thread configuration often became a weak link in the rod string under more demanding conditions.

Shock and impact forces encountered with gas or fluid pounding appeared to be the most frequent failure consideration, because these forces excessively stressed the polish rod connection threads as they alone absorbed longitudinal stresses and impact forces on the rod.

Such breakdowns are aggravated because, it is well known, movement between threaded members tends to destroy the connection.

Polish rod connection failures were evidenced, for both beam pumping and progressive cavity pumping, by “belling out” (radial expansion) of the coupling due to the prior sucker rod design.

In rotary systems gas absorbed in the stator elastomer, results in torque increasing, which may be accompanied by torque spikes caused by the ingestion, by the pump of formation solids, with failures like those mentioned above.

Images